The invention relates to cutter assemblies for rotary drill bits for use in drilling or coring holes in subsurface formations.
The cutter assemblies are for use in rotary drill bits of the "drag" kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and/or cooling the cutters.
Each cutter assembly comprises a stud which is received in a socket in the surface of the bit body, the stud being formed, adjacent one end thereof, with a plane surface disposed at an angle of less than 90.degree. to the longitudinal axis of the stud, and a preform cutting element being mounted on the inclined surface of the stud. The preform cutting element may be of the kind comprising a tablet, usually circular, having a thin hard cutting table of polycrystalline diamond bonded to a thicker, less hard substrate, for example of tungsten carbide. The two-layer arrangement of the cutting element provides a degree of self-sharpening since, in use, the less hard substrate wears away more easily than the harder cutting table. However single layer preform cutting elements of thermally stable polycrystalline diamond are also known.
The preform cutting elements are usually mounted on the stud by brazing a flat rear surface of the cutting element to the inclined surface of the stud. This arrangement normally provides a junction between the peripheral edge surface of the cutting element and the inclined plane surface of the stud which is, as seen in section, a sharp right-angled junction. It appears that this arrangement results in a stress concentration at the junction, particularly in the region furthest from the cutting edge of the cutting element, as a result of forces applied to the cutter assembly in use. This stress concentration is found to reduce the bending strength of the cutter assembly since it may initiate cracking of the stud in the area of the junction. Cutter assemblies of this kind are found to have a tendency to fail as a result of crack propagation through the stud from this area.
The present invention sets out to provide constructions of cutter assemblies, and methods of manufacture, which may increase the bending strength of the assembly and reduce this tendency of the stud to fail in this manner.